This relates generally to image sensors, and more specifically, to image sensors operable in global shutter mode.
Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. Conventional image sensors are fabricated on a semiconductor substrate using complementary metal-oxide-semiconductor (CMOS) technology or charge-coupled device (CCD) technology. The image sensors may include an array of image sensor pixels each of which includes a photodiode and other operational circuitry such as transistors formed in the substrate.
A dielectric stack is formed on the substrate over the photodiodes. The dielectric stack includes metal routing lines and metal vias formed in dielectric material. Light guides are often formed in the dielectric stack to guide the trajectory of incoming light. A color filter array is typically formed over the dielectric stack to provide each pixel with sensitivity to a certain range of wavelengths. Microlenses are formed over the color filter array. Light enters the microlenses and travels through the color filters into the dielectric stack.
In a conventional image sensor configured to operate in global shutter mode, each image sensor pixel includes a photodiode for detecting incoming light and a separate storage diode for temporarily storing charge. The storage diode should not be exposed to incoming light. In such arrangements, structures such as tungsten buried light shields (abbreviated as WBLS) are formed on the substrate between neighboring photodiodes to help prevent stray light from affecting the storage diode. At least some metal vias are formed through gaps in the buried light shields in order to control pixel transistors formed between two adjacent photodiodes. Shielding storage diodes in this way can help reduce crosstalk and increase global shutter efficiency (i.e., the buried light shields are designed to prevent stray light from entering regions of the substrate located between two adjacent photodiodes).
In practice, however, the tungsten buried light shield reflects stray light. The reflected stray light may be scattered through the existing gaps in the buried light shield and corrupt the storage diode. Moreover, charge generated at the edges of the photodiode may sometimes inadvertently leak to adjacent storage diodes. This results in undesirable pixel crosstalk and degraded global shutter efficiency.
It is within this context that the embodiments described herein arise.